Radiation-sensitive compositions are routinely used in the preparation of imagable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, a radically polymerizable component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of from about 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and non-exposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the non-exposed regions become an image.
Various negative-working radiation compositions and imagable elements containing reactive polymer binders are known in the art. Some of these compositions and elements are described for example in U.S. Pat. No. 6,569,603 (Furukawa), U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,582,882 (Pappas et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), and U.S. Pat. No. 6,787,281 (Tao et al.), U.S. Patent Application Publication 2003/0118939 (West et al.), and EP 1,079,276A1 (Lifka et al.), EP 1,182,033A1 (Fujimaki et al.), and EP 1,449,650A1 (Goto).
“On-press” development technology has become more prominent in recent years to avoid the use of traditional developers. On-press developable imagable elements that have been designed for this purpose are described, for example in U.S. Pat. No. 6,582,882 (Hayashi et al.), U.S. Pat. No. 6,899,994 (Huang et al.), and U.S. Pat. No. 7,005,234 (Hoshi et al.) and U.S. Patent Published Applications 2005/003285 (Hayashi et al.), 2005/026302 (Mitsumoto et al.), and 2006/0057492 (Kunita et al.). Such elements can be directly mounted on a press after imaging, and developed through contact with a lithographic printing ink, fountain solution, or both. Thus, a separate development step after imaging using a traditional developer is avoided. On-press imaging, in which the imagable element is both imaged and developed on-press, eliminates the need to mount the element in a separate imaging device.
Some negative-working compositions and imagable elements include iodonium borate salts in the initiator compositions as described for example, in copending and commonly assigned U.S. Ser. Nos. 11/138,026 (filed May 26, 2005 by Knight et al.), 11/356,518 (filed Feb. 17, 2006 by Tao et al.), and 11/349,376 (filed Feb. 7, 2006 by Tao et al.).
JP Kokai (published unexamined applications) 2002-116539 and 2005-062482 describe the use of borates, such as tetraarylborates, as initiators in negative-working compositions and imagable elements for making lithographic printing plates. These compounds are present as “free salts”, meaning that they are not purposely bound to polymers. Polyborates are described as co-initiators in photopolymerizable composition in U.S. Pat. No. 6,087,062 (Cunningham et al.).